1. Field of the Invention
The invention relates to an electric power steering device.
2. Description of Related Art
A conventional column-assist type electric power steering device (which is hereinafter referred to as “EPS”) that is tiltable will be described with reference to FIG. 5. The EPS includes a steering column 100 and a steering shaft 102 that is rotatably inserted in the steering column 100, and a housing 104 is integrally connected to a front part of the steering column 100.
The housing 104 has an axial hole 106 in which an output shaft 108, which constitutes a part of the steering shaft 102, is rotatably supported via bearings 110, 112 and 114. A worm wheel 116 is attached to and press-fitted to the output shaft 108 such that the worm wheel 116 is unable to rotate relative to the output shaft 108, for example, through a key connection, and the worm wheel 116 meshes with a worm 118 that is connected to an output shaft of a motor (not shown). The bearings 110, 112 and 114 are attached to the axial hole 106 by press-fitting.
Thus, the steering force that is generated when a driver operates a steering wheel (not shown) that is connected to the steering shaft 102 is transmitted via the steering shaft 102 to the output shaft 108 and then to steered wheels (not shown) via, for example, a rack-and-pinion steering device. In addition, the rotary force of the motor is transmitted via the worm 118 and the worm wheel 116 to the output shaft 108. As a result, by appropriately controlling the rotary force and direction of rotation of the motor, appropriate steering assisting torque can be provided to the output shaft 108 to achieve power assistance.
An electric power steering device that has the above-described structure, which is described in, for example, Japanese Patent Application Publication No. 2014-156177 (JP 2014-156177 A), is known.
When a steered wheel (tire) goes up onto a curb or the like violently while the vehicle is running, the motor and the steering wheel receive an excessive reverse input from the steered wheel and are rotated at a high speed. When an excessive reverse input is applied as described above, the worm wheel, which is connected to the output shaft by press-fitting, may be released from the output shaft and moved in an axial direction of the output shaft. When the worm wheel and the worm are disengaged from each other as a result of the movement of the worm wheel, there is a possibility that the electric power steering device becomes unable to provide power assistance.